1. Field of the Invention
The present invention relates to a cellular radio communication system and, in particular, to a method and apparatus for defining transmission/reception timing of physical channel in cross-carrier scheduling of TDD system supporting carrier aggregation.
2. Description of the Related Art
Recently, there are many researches being conducted on the Orthogonal Frequency Division Multiple Access (OFDMA) and Single Carrier-Frequency Division multiple Access (SC-FDMA) in the cellular communication field. Such a multiple access technology is used to allocate and manage the time-frequency resources for data and/or control information transmission to and from multiple users without overlapping from each other, i.e. orthogonally, so as to makes it possible to discriminate among per-user data and control informations.
One of the significant features of the cellular communication system is to support scalable bandwidth for providing high speed wireless data service. For example, the Long Term Evolution (LTE) system is capable of supporting various bandwidths, e.g. 20/15/5/3/1.4 Mhz. The mobile carriers can provide their services with one of the available bandwidths. A User Equipment (UE) can operate with various capabilities of minimum 1.4 MHz bandwidth up to 20 MHz bandwidth. Meanwhile, the LTE-Advanced (hereinafter, called LTE-A) system can support high data rate transmission over a wide bandwidth up to 100 MHz for a single UE with carrier aggregation.
In order to support the high data rate transmission, the LTE-A system requires the bandwidth wider than that of the LTE system while preserving backward compatibility to the legacy systems for supporting the LTE UEs. For the backward compatibility, the system bandwidth of the LTE-A system is divided into a plurality of subbands or component carriers (CC) that can be used for transmission/reception of LTE UEs and aggregated for the high data rate transmission of the LTE-A system with the transmission/reception process of the legacy LTE system per component carrier.
The scheduling information for the data transmitted on the component carriers is transmitted to the UE in Downlink Control Information (DCI). The DCI is generated in different DCI format according to whether scheduling information is of uplink or downlink, whether the DCI is compact DCI, whether spatial multiplexing with multiple antennas is applied, and whether the DCI is the power control DCI. For example, the DCI format 1 for the control information about downlink data to which Multiple Input Multiple Output (MIMO) is not applied is composed of the following control informations.                Resource allocation type 0/1 flag: It notifies the UE of whether the resource allocation type is type 0 or type 1. Here, type 0 indicates resource allocation in unit of resource block group (RBG) in bitmap method. In LTE and LTE-A systems, the basic scheduling unit is resource block (RB) representing time and frequency resource, and RBG is composed of a plurality of RBs and basic scheduling unit of in type 0. Type 1 indicates allocation of specific RB in RBG.        Resource block assignment: It notifies the UE of RB allocated for data transmission. At this time, the resource expressed according to the system bandwidth and resource allocation scheme is determined.        Modulation and coding scheme: It notifies the UE of modulation scheme and coding rate applied for data transmission.        HARQ process number: it notifies the UE of HARQ process number.        New data indicator: It notifies the UE of whether the transmission is        HARQ initial transmission or retransmission.        Redundancy version: It notifies the UE of redundancy version of HARQ.        TPC command for PUCCH: It notifies the UE of power control command for Physical Uplink Control Channel (PUCCH) as uplink control channel.        
The DCI is channel-coded and modulated and then transmitted through Physical Downlink Control Channel (PDCCH).
FIG. 1 is a diagram illustrating an exemplary case where an eNB schedules downlink data for a UE with two aggregated carriers (CC#1, CC#2) in the LTE system. In FIG. 1, the DCI 1 101 to be transmitted on the Component Carrier #1 (CC#1) 109 is generated with a format defined in the legacy LTE and channel coded and interleaved so as to be carried by PDCCH 103. The PDCCH 103 carries the scheduling information about the Physical Downlink Shared Channel 213 as the data channel allocated to the UE on the CC#1 109. The DCI 105 transmitted on the component carrier #2 (CC#2) 111 is formatted as defined in the legacy LTE standard, channel-coded, and then interleaved to generate PDCCH 107. The PDCCH 107 carries the scheduling information about the PDSCH 115 as the data channel allocated to the UE on the CC#2 111.
In the LTE-A system supporting carrier aggregation, the data and/or DCI for supporting the data transmission can be transmitted per component carrier as shown in FIG. 1. In case of DCI, however, it can be transmitted on another component carrier different from the component carrier carrying the data, and this is referred to as cross-carrier scheduling. That is, the scheduling information about the data transmitted on the CC#2 is transmitted on the CC#1. The cross-carrier scheduling is described hereinafter in detail with reference to FIG. 2.
FIG. 2 is a diagram illustrating a procedure for scheduling an LTE-A UE using aggregated component carriers #1 and #2 (CC#1 and CC#2) 209 and 219. FIG. 2 is directed to an exemplary case where the CC#2 219 experiences significant interference as compared to CC#1 209 such that it is difficult to satisfy a predetermined DCI reception performance requirement for data transmission on the CC#2 219. In this case, the eNB may transmit the DCI on the CC#1 219. Since any error occurring in data transmission can be corrected later through HARQ, there is no problem in transmitting data on the CC#2 although significant interference exists thereon. In order to make it possible to operate as above, it is necessary for the eNB to transmit a carrier indicator (CI) indicating the component carrier targeted by the DCI along with the DCI indicating the resource allocation information and transmission format of the scheduled data. For example, CI=‘00’ indicates CC#1 209 and, CI=‘01’ indicates CC#2 219.
Accordingly, the eNB combines the DCI 201 indicating resource allocation information and transmission format of the scheduled data 207 and the carrier indicator 202 to generate an extended DCI, performs channel coding, modulation, and interleaving on the extended DCI to generate PDCCH, and maps the PDCCH to the PDCCH region 205 of CC#1 209. The eNB also combines the DCI 211 indicating the resource allocation information and transmission format of the data 217 scheduled on CC#2 and the carrier indicator 212 to generate an extended DCI, performs channel coding, modulation and interleaving on the extended DCI to generate PDCCH, and maps the PDCCH to the PDCCH region 205 of CC#1 209.
The TDD system uses a common frequency for uplink and downlink which are discriminated in time domain. In the LTE TDD system, the uplink and downlink signals are discriminated by subframe. A radio frame can be divided into equal number of uplink and downlink subframes according to the uplink and downlink traffic load, but the number of uplink subframes may greater than that of the downlink subframes and vice versa. In the LTE system, the subframe has a length of 1 ms, 10 subframes form a radio frame.
TABLE 1Uplink-downlinkSubframe numberconfiguration01234567890DSuuuDSuuu1DSuuDDSuuD2DSuDDDSuDD3DSuuuDDDDD4DSuuDDDDDD5DSuDDDDDDD6DSuuuDSuuD
Table 1 shows TDD configurations (TDD uplink-downlink configurations) defined in LTE standard. In table 1, subframe numbers 0 to 9 indicates the indices of subframes constituting one radio frame. Here, ‘D’ denotes a subframe reserved for downlink transmission, ‘U’ denotes a subframe reserved for uplink transmission, and ‘S’ denotes the special subframe. The DwPTS can carry the downlink control information as the normal subframe does. If the DwPTS is long enough according to the configuration state of the special subframe, it is possible to carry the downlink data too. The GP is the interval required for downlink-to-uplink switch and its length is determined according to the network configuration. The UpPTS can be used for transmitting UE's Sounding Reference Signal (SRS) for uplink channel state estimation and UE's Random Access Channel (RACH).
For example, in case of TDD uplink-downlink configuration#6, the eNB can transmit downlink data and/or control information at subframes #0, #5, and #9 and uplink data and/control information at subframes #2, #3, #4, #7, and #8. Here, # indicates number or index. The subframes #1 and #6 as special subframes can be used for transmitting downlink control information and/or downlink data selectively and SRS or RACH in uplink.
Since the downlink or uplink transmission is allowed for specific time duration in the TDD system, it is necessary to define the timing relationship among the uplink and downlink physical channels such as control channel for data scheduling, scheduled data channel, and HARQ ACK/NACK channel (HARQ acknowledgement) corresponding to the data channel.
In the TDD system, the timing relationship between Physical Downlink Shared Channel (PDSCH) and Physical Uplink Control channel (PUCCH) carrying uplink HARQ ACK/NACK corresponding to the PDSCH or Physical Uplink Shared Channel (PUSCH) is as follows.
The UE receives the PDSCH transmitted by the eNB at (n−k)th subframe and transmits uplink HARQ ACK/NACK corresponding to the received PDSCH at nth subframe. Here, k denotes an element of a set K, and K is defined as shown in table 2.
TABLEUL-DLSubframenConfiguration01234567890——6—4——6—41——7, 64———7, 64—2——8, 7, 4, 6————8, 7, 4, 6——3——7, 6, 116, 55, 4—————4——12, 8, 7, 116, 5, 4, 7——————5——13, 12, 9, 8, 7, 5,———————4, 11, 66——775——77—
FIG. 3 is a diagram illustrating a timing relationship between PDSCH and uplink HARQ ACK/NACK to show which subframe carries uplink HARQ ACK/NACK corresponding to PDSCH that is transmitted in a downlink subframe or a special subframe in TDD uplink-downlink configuration 6 as defined in table 2. For example, the UE transmits, at subframe #7 of ith radio frame, the uplink HARQ ACK/NACK corresponding to the PDSCH 301 transmitted by the eNB at subframe #1 of ith subframe. At this time, the downlink control information (DCI) including the scheduling information on the PDSCH 301 is transmitted through PDCCH of the subframe which also carries PDSCH. For another example, the UE transmits, at the subframe #4 307 of (i+1)th radio frame, the uplink HARQ ACK/NACK corresponding to PDSCH 305 transmitted by the eNB at subframe #9 of the ith radio frame. Likewise, the downlink control information (DCI) including the scheduling information on the PDSCH 305 is transmitted through PDCCH of the subframe which also carries PDSCH.
The LTE adopts an asynchronous HARQ in downlink in which the data retransmission timing is not fixed. That is, when an HARQ ACK fed back by the UE in response to the HARQ initial transmission data transmitted by the eNB is received, the eNB determines the next HARQ retransmission timing freely according to the scheduling operation. The UE buffers the data failed in decoding for HARQ operation and combines the buffered data with the next HARQ retransmission data. In order to keep the reception buffer space to a predetermined level, a maximum number of HARQ processes are defined per TDD uplink-downlink configuration as shown in table 3. One HARQ process is mapped to one subframe in time domain.
TABLE 3TDD UL/DL configurationMaximum number of HARQ processes04172103941251566
Referring to table 3, if it fails to decode the PDSCH 301 transmitted by the eNB at subframe #0 of the ith radio frame, the UE transmits an HARQ NACK at the subframe #7 of ith radio frame. Upon receipt of the HARQ NACK, the eNB configures the retransmission data corresponding to PDSCH 301 as PDSCH 309 and transmits the PDSCH 309 along with PDCCH. In the exemplary case of FIG. 3, the retransmission data is transmitted in the subframe #1 of (i+1)th radio frame by taking notice that the maximum number of downlink HARQ processes is 6 in the TDD uplink-downlink configuration #6 according to the definition of table 3. This means that there are total 6 downlink HARQ processes 311, 312, 313, 314, 315, and 316 between the initial transmission, i.e. PDSCH 301, and the retransmission, i.e. PDSCH 309.
The LTE system adopts synchronous HARQ having fixed data transmission points in uplink unlike the downlink HARQ. That is, the uplink/downlink timing relationship among the Physical Uplink Shared Channel (PUSCH), Physical Downlink Control Channel (PDCCH) followed by the PUSCH, and Physical Hybrid Indicator Channel (PHICH) carrying the downlink HARQ ACK/NACK corresponding to the PUSCH are fixed according to a rule as follows.
If the PDCCH including DCI format 0 as uplink scheduling control information or the PHICH carrying the downlink HARQ ACK/NACK is received from the eNB at nth subframe, the UE transmits the PUSCH carrying uplink data corresponding to the control information at (n+k)th subframe. Here, k is denoted as shown in table 4.
TABLE 4TDDUL/DLDL subframe number nConfiguratio01234567890464616464244344444454677775
If the PHICH carrying downlink HARQ ACK/NACK is received from the eNB at ith subframe, the PHICH corresponds to the PUSCH transmitted by the UE at (i+k)th subframe. Here, k is defined as shown in table 5.
TABLE 5TDDUL/DLDL subframe number iConfiguratio01234567890747414646266366646656664746
FIG. 4 is a diagram illustrating the subframes carrying PUSCH corresponding to the PDCCH or PHICH carried in downlink or special subframe and the subframes carrying PHICH corresponding to the PUSCH according to the definition of tables 4 and 5 in case of TDD uplink-downlink configuration #1. For example, the PUSCH corresponding to the PDCCH or PHICH 401 transmitted by the eNB at subframe #1 of ith radio frame is transmitted by the UE at subframe #7 of ith radio frame (403). The eNB transmits PHICH to the UE at subframe #1 of (i+1)th radio frame (405). For another example, the PUSCH corresponding to PDCCH or PHICH 407 transmitted by the eNB at subframe #6 of ith subframe is transmitted by the UE at subframe #2 of (i+1)th radio frame (409). The eNB transmits PHICH corresponding to the PUSCH to the UE at subframe #6 of (i+1)th radio frame (411).
In association with PUSCH transmission in LTE TDD system, the downlink transmission of PDCCH or PHICH corresponding to PUSCH is restricted a specific downlink subframe to guarantee minimum transmission/reception processing time of the eNB and UE. For example, in case of the TDD downlink-uplink configuration #1 of FIG. 4, the PDCCH for PUSCH scheduling or the PHICH corresponding to PUSCH is muted at subframes #0 and #5.
In case of adopting the timing relationship among the physical channels that is designed for the LTE TDD system to the LTE-A system, there is a need of defining extra operation in addition to the legacy timing relationship. In detail, if the cross-carrier scheduling is applied for the case where TDD uplink-downlink configurations of the respective aggregated carriers differ from each other, it is required to define timing relationships between PDCCH and cross-carrier scheduled PDCCH, between cross-carrier scheduled PUSCH and PHICH, and between cross-scheduled PDSCH and uplink HARQ ACK/NACK.